Investigations with cats and monkeys have shown that binocular anomalies present during the critical period of development may result in loss of binocularity, including stereopsis, and the development of amblyopia. The extent to which the visual system can recover from these deficits depends, in part, on the time and adequacy of correction during the critical period. The implication of the animal research for human binocular anomalies is theoretically straightforward. If the patient can be treated at the proper time during the critical period of development, his prognosis for the development of normal visual functioning may be made more favorable. Although a critical period for development of the human visual system is recognized, its course is not clearly delineated. The major objective of the proposed research will be to ascertain the timing, and variations in the sensitivity, of the period of susceptibility of the human visual system to modification by unilateral occlusion and misaligned visual axes. We shall try to define the relationship between the loss of binocularity and the development of amblyopia. It is our intent to use both retrospective and prospective studies to achieve these ends. The outcome should be relevant to determining the optimal time for surgical intervention and for strabismic therapy. The proposed research is a natural extension of on-going work in our laboratory which deals with the development of vision in normal and astigmatic infants.